Method and device for ensuring storage time for digital broadcast

ABSTRACT

When a digitized broadcast is recorded in a storage medium, a required capacity of the storage medium is firstly calculated on the basis of a bit-rate of digitized broadcast data and a recording time which is known in advance. If the remaining capacity of the storage medium is smaller than the required capacity of the storage medium, for example, a frame rate decimation and pixel reduction are performed upon a broadcast to be newly stored, a re-encoding processing is performed for the broadcast and then the resultant broadcast is stored in the storage medium. If the remaining storage capacity of the storage medium is larger than the required capacity of the storage medium and thus remains, a frame interpolation and pixel interpolation are performed for the broadcast to be stored and then a re-encoding processing is performed for the resultant broadcast in order to improve its image quality. As a result, recording of the broadcast data in the storage medium can be ensured.

BACKGROUND OF THE INVENTION

The present invention relates to a method and a device for ensuringstorage time for digital broadcast that can ensure recording timerequired when a digital broadcast compressed by MPEG or the like isstored and recorded in a recording medium such as an optical disk, anoptomagnetic disk or a hard disk.

Recently, in accordance with improvement of image signal processingtechniques, a record-and-playback device for recording in a disk-shapedstorage medium that is capable of freely recording/playing back a videoor voice digital signal, such as a DVD-RAM or a hard disk has beendeveloped.

As a method for storing and recording analog broadcast data in such astorage medium, for example, as disclosed in Japanese Patent ApplicationLaid-Open (JP-A) No. 8-17137, there has been known a method that atarget bit-rate is calculated on the basis of the storage capacity ofthe storage medium and a predetermined recording time and then the datais encoded so as to be contained in the storage medium. Namely, becausethe recording capacity for the storage medium and the recording time foran analog broadcast to be stored are known, a bit-rate can be determinedby the expression, recording capacity÷recording time. Then, control isperformed so that the analog broadcast is compressed and recorded atthis resultant bit-rate.

In the case of storing and recording not analog broadcast data butdigital broadcast data, however, sources for the digital broadcast arefor a standard television in current CS broadcasting performed in Japan.In BS digital broadcasting, a standard television and high-definitiontelevision are supported. Further, even in the same standard televisionor high-definition television, a bit-rate of broadcast is different foreach program. In ground wave digital broadcasting that will be startedin 2003 or later, a standard television, a high-definition televisionand a level lower than MPEG4 Simple Profile Level 3 will be provided. Inaccordance with MPEG4 Simple Profile Level 3, an image size is CIF (¼ ofcurrent vertical and horizontal sizes, respectively) and a bit-rate isequal to or less than 384 kbit/s. Thus, when these three kinds ofbroadcasts are stored and recorded in a digital form, a storage time maybe varied for each of the bit-rates for these broadcasts even in thecase of storing the same broadcast program, unlike a conventional analogVHS VTR which can simply define the storage time in a 120 min tape astwo hours in a standard mode and six hours in a triple-mode.Accordingly, a problem exists in the method for ensuring a storing andrecording time.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method and a devicefor ensuring the time for digital broadcasts transmitted at variousbit-rates to be recorded in a storage medium.

In order to accomplish the aforementioned object, in accordance with thepresent invention, when a digitized and then transmitted broadcast isstored and recorded, the capacity of a storage medium required forstoring the broadcast is calculated. Then, the required capacity iscompared to an actual empty area in the storage medium, a bit-rate ofthe broadcast to be stored is appropriately converted, and the resultantbroadcast is stored.

According to the present invention, a method for ensuring a storage timefor digital broadcast at the time of storing data for a digitized andthen transmitted broadcast comprises the steps of: monitoring apredetermined recording time for the broadcast and a bit-rate of thebroadcast and calculating, in real time, a required capacity of astorage medium; determining whether or not the calculated requiredcapacity of the storage medium can be contained within an empty area ora freely limited area of the storage medium; if the calculated requiredcapacity of the storage medium cannot be contained as the result of thedetermination, converting, in real time, the bit-rate of the broadcastinto a smaller one; and storing the converted broadcast in the storagemedium.

In accordance with the present invention, the method for ensuring astorage time for digital broadcast further comprises the steps of: ifthe calculated required capacity of the storage medium can be containedas the result of the determination, determining whether or not the emptyarea or freely limited area of the storage medium remains; if the emptyarea or freely limited area remains, converting, in real time, thebit-rate of the broadcast into a larger one; and storing the convertedbroadcast into the storage medium.

According to the present invention, the method for ensuring a storagetime for digital broadcast further comprises the step of: if thecalculated required capacity of the storage medium can be contained asthe result of the determination, storing the broadcast in the storagemedium without converting the bit-rate.

According to the present invention, in the method for ensuring a storagetime for digital broadcast, the step of converting the bit-rate of thedigitized broadcast into a smaller one is an operation of missing a partof the broadcast data.

In accordance with the method for ensuring a storage time for digitalbroadcast of the present invention, the step of converting the bit-rateof the digitized broadcast into a smaller one is an operation ofreturning the broadcast in a baseband and performing a compressionprocessing again for the broadcast.

In accordance with the method for ensuring a storage time for digitalbroadcast of the present invention, the operation of returning thebroadcast in a baseband and performing a compression processing againfor the broadcast is performed by utilizing a system for compressing ananalog broadcast, decoding the compressed digital data and then playingback the resultant data.

According to the present invention, in the method for ensuring a storagetime for digital broadcast, the step of converting the bit-rate of thedigitized broadcast into a smaller one is an operation of returning thebroadcast in a baseband, then performing image resolution conversion orimage frame rate conversion upon the broadcast and performing thecompression processing again upon the resultant broadcast.

According to the present invention, in the method for ensuring a storagetime for digital broadcast, the step of converting the bit-rate of thedigitized broadcast into a smaller one is performed by selecting onefrom three kinds of operations, i.e., the operation of missing a part ofthe broadcast data, the operation of returning the broadcast in abaseband and performing a compression processing again upon thebroadcast and the operation of returning the broadcast in a baseband,then performing image resolution conversion or image frame rateconversion upon the broadcast and performing the compression processingagain upon the resultant broadcast.

The method for ensuring a storage time for digital broadcast of thepresent invention further comprises the step of: if the calculatedrequired capacity of the storage medium cannot be contained, deleting abroadcast which has been already stored in the storage medium to ensurethe required capacity of the storage medium.

The method for ensuring a storage time for digital broadcast of thepresent invention further comprises the step of: if the calculatedrequired capacity of the storage medium cannot be contained,re-compressing the broadcast which has been already stored in thestorage medium to ensure the required capacity of the storage medium.

In accordance with the present invention, in the method for ensuring astorage time for digital broadcast, the step of re-compressing thebroadcast which has been already stored in the storage medium isperformed by the operation of missing a part of the stored broadcastdata, the operation of returning the stored broadcast in a baseband andperforming a compression processing again upon the broadcast or theoperation of returning the stored broadcast in a baseband, thenperforming image resolution conversion or image frame rate conversionand performing a compression processing again upon the broadcast.

In accordance with the present invention, in the method for ensuring astorage time for digital broadcast, when a plurality of already storedbroadcasts exist in the storage medium, a broadcast that a set period oftime or longer has passed since its storage will be deleted.

In accordance with the present invention, in the method for ensuring astorage time for digital broadcast, when a plurality of already storedbroadcasts exist in the storage medium, a broadcast which has beendecoded at least once since its storage will be deleted.

In accordance with the present invention, in the method for ensuring astorage time for digital broadcast, when a plurality of already storedbroadcasts exist in the storage medium, a broadcast which is stored inthe storage medium and then copied in other storage medium will bedeleted.

In accordance with the present invention, in the method for ensuring astorage time for digital broadcast, when a plurality of already storedbroadcasts exist in the storage medium, a broadcast which was stored atthe earliest date will be deleted.

In accordance with the present invention, in the method for ensuring astorage time for digital broadcast, when a plurality of already storedbroadcasts exist in the storage medium, a broadcast that a set period oftime or longer has passed since its storage, a broadcast which has beendecoded at least once since its storage, a broadcast which is stored inthe storage medium and then copied in other storage medium or abroadcast which was stored at the earliest date will be deleted, andthese broadcasts are selected in accordance with a predeterminedpriority.

According to the present invention, a device for ensuring a storage timefor digital broadcast at the time of storing a digitized and thentransmitted broadcast comprises a required capacity calculation circuitfor monitoring a predetermined recording time of the broadcast and abit-rate of the broadcast and calculating, in real time, a requiredcapacity of the storage medium; a determination circuit for determiningwhether or not the required capacity of the storage medium calculated inthe required capacity calculation circuit can be contained within anempty or freely limited area in the storage medium; and a bit-rateconversion circuit for, if the calculated required capacity of thestorage medium cannot be contained as the result of the determination bythe determination circuit, converting, in real time, the bit-rate of thebroadcast into a smaller one and storing the converted broadcast in thestorage medium.

According to the present invention, in the device for ensuring a storagetime for digital broadcast, the determination circuit furtherdetermines, if the calculated required capacity of the storage mediumcan be contained, whether or not the empty or freely limited area of thestorage medium remains. The device further comprises a second bit-ratechanging circuit for, if the empty or freely limited area remains as theresult of the determination by the determination circuit, converting, inreal time, the bit-rate of the broadcast into a larger one and storingthe converted broadcast in the storage medium.

In the device for ensuring a storage time for digital broadcast of thepresent invention, the required capacity calculation circuit comprises adetermination circuit for writing an inputted broadcast stream in astorage memory and determining the bit-rate of the broadcast on thebasis of its storage time and storage amount.

The device for ensuring a storage time for digital broadcast of thepresent invention further comprises a deletion circuit for, if thecalculated required capacity of the storage medium cannot be containedas the result of determination by the determination circuit, deleting abroadcast which has been already stored in the storage medium.

The device for ensuring a storage time for digital broadcast of thepresent invention further comprises a re-compression circuit for, if thecalculated required capacity of the storage medium cannot be containedas the result of determination by the determination circuit,re-compressing a broadcast which has been already stored in the storagemedium.

In the device for ensuring a storage time for digital broadcast of thepresent invention, the required capacity calculation circuit, thedetermination circuit and the bit-rate conversion circuit are integratedonto a chip.

As described above, in accordance with the present invention, even ifthe bit-rate of a digitized broadcast is varied, a required capacity ofa storage medium at an original bit-rate of the broadcast issuccessively calculated. If the required capacity of the storage mediumcannot be contained within an empty or freely limited area in thestorage medium, the bit-rate of the broadcast is converted into asmaller one. Thus, the capacity of the storage medium required for thebroadcast can be contained within the remaining storage capacity and thebroadcast can be reliably stored and recorded in the storage medium.

Especially in the present invention, if the required capacity of thestorage medium can be successfully contained within the remainingstorage capacity, the transmission bit-rate of the broadcast isconverted into a larger one and the resultant broadcast is stored in thestorage medium. Thus, the broadcast can be stored and recorded with highimage quality.

The present invention can be provided inexpensively because are-compression processing is performed for a broadcast to be newlystored by utilizing existing devices in a storage-and-playback system.

Further, in accordance with the present invention, the required capacityof the storage medium is ensured in an empty area therein by deleting orre-compressing a broadcast which has been already stored in the storagemedium. Thus, while maintaining at a bit-rate required for successfullymaintaining a played back image, the broadcast to be newly stored can bereliably stored and recorded in the storage medium. In accordance withthe present invention, among a plurality of already stored broadcasts, abroadcast that a set period of time has passed since its storage or abroadcast that is at least decoded or copied is preferentially deletedor re-compressed. As a result, the already stored broadcast can bedeleted or re-compressed depending on a degree of necessity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a device for ensuring a storagetime for digital broadcast relating to embodiments of the presentinvention.

FIG. 2 is a flowchart illustrating the detail of a method for ensuringthe storage time performed by the device for ensuring storage time fordigital broadcast.

FIG. 3 is a view illustrating the algorithm for establishing thepriority of a broadcast to be deleted or re-compressed.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of the present invention will be describedhereinafter with reference to the attached drawings.

FIG. 1 is a block diagram illustrating the specific structure of adevice for ensuring storage time for digital broadcast relating to theembodiments of the present invention.

The device for ensuring storage time for digital broadcast illustratedin FIG. 1 is mainly configured by an input section 1, an MPEG transportdecoder 17, a stream controller 6, an MPEG decoder 10, an imageresolution/frame rate converter 9, an MPEG encoder 11, a drive section16 and a microcomputer 14 for entirely controlling these elements.

Inputted to the input section 1 is a broadcast signal MPEG-TS 2 obtainedby multiplexing at least one broadcast program inputted from a tunersection of a satellite, cable television or ground wave digitalbroadcasting, such as an MPEG-TS signal. The input section 1 outputs themultiplexed TS bit stream 3 to the MPEG transport decoder 17. The MPEGtransport decoder 17 selects only a broadcast program selected inaccordance with information about the selected program designated inadvance by the microcomputer 14, and outputs the TS bit stream 4 of theselected program. The total storage time for the TS bit stream 4 of theselected broadcast program is inputted in advance by various methodsincluding G-Code, EPG, and user's time setting and the like, and thus isknown by the microcomputer 14 prior to storing and recording of thebroadcast program.

The TS bit stream 4 of the selected broadcast program outputted from theMPEG transport decoder 17 is inputted to the stream controller 6. Thisstream controller 6 stores and records the inputted TS bit stream 4 ofthe broadcast program in a storage medium driven by the drive section 16such as DVD-R, DVD-RAM, DVD-RW, Blu-Ray disk or contained hard disk.This stream controller 6 has the function of determining a bit-rate ofthe TS bit stream 4 of the broadcast program to be stored prior to thebroadcast program being stored in the storage medium. In order toexhibit this function, the stream controller 6 has an inputted bit-ratedeterminer (determination circuit) 19 therein, and an external buffermemory 5 is connected thereto. The stream controller 6 starts to storethe inputted TS bit stream 4 of the selected broadcast program in theexternal buffer memory (storage memory) 5 before outputting the same tothe drive section 16. Under this state, the inputted bit-rate determiner(determination circuit) 19 monitors, in real time, the bit-rate of theTS bit stream 4 of the selected broadcast program on the basis of astorage time and storage amount in the external buffer memory 5, andoutputs the determined bit-rate to the microcomputer 14.

The microcomputer 14 determines a required capacity of the storagemedium on the basis of the determined bit-rate and the storing andrecording time for the broadcast program to be stored, compares therequired capacity of the storage medium to an empty area in the storagemedium in the drive section 16 or a freely limited area determined inthe microcomputer 14 by a user (which will be referred to as a remainingstorage capacity hereinafter), and determines whether or not thebroadcast program to be stored can be stored and recorded in the storagemedium. The specific example of such determination will be describedwith reference to FIG. 2.

FIG. 2 illustrates a flowchart of the determination and processingsafter the determination. Referring to FIG. 2, firstly in step S0, thestorage time for the broadcast program to be stored is multiplied by thebit-rate monitored in real time by the bit-rate determiner 19 in orderto calculate a required capacity of the storage medium for the digitizedbroadcast program. A required capacity calculation circuit 50 isconfigured by step S0.

Next, in step S1, the calculated required capacity of the storage medium(result of multiplication) is compared with the remaining storagecapacity of the storage medium to determine the magnitude relationshipbetween the required capacity of the storage medium and the remainingstorage capacity thereof. Namely, it is determined whether or not therequired capacity of the storage medium is smaller than the remainingstorage capacity and thus the broadcast program to be stored can bestored in the storage medium. A determination circuit 51 is configuredby step S1.

If the remaining capacity of the storage medium is smaller as the resultof comparison of magnitude, the process proceeds to step S2. In step S2,a determination is made as to whether the broadcast program to be storedis re-encoded at smaller bit-rate. If the re-encoding processing isperformed, this re-encoding processing is performed by two kinds ofmethods. In accordance with one method, the broadcast to be stored isdecoded in a compressed database band and then encoded again. Inaccordance with the other method, a part of bit stream of the broadcastis deleted in order to obtain a predetermined small bit-rate.Specifically explaining, in step S4, a determination is made as towhether the broadcast is subjected to the decoding processing to bereturned in a baseband. If the answer to the determination in step S4 isNO., in step S5, a part of bits of the broadcast program to be stored isdeleted so as to be missed therefrom. Various methods are provided forthis deletion and may include, e.g., a method for deleting bitssubjected to padding processing for controlling a fixed rate. If theanswer to the determination as to whether the broadcast is returned in abaseband is YES, in step S6, whether or not a resolutionconversion/frame rate decimation is performed is determined. If theanswer to the determination in step S6 is NO, in step S7, the broadcastprogram returned in a baseband is subjected to the re-encodingprocessing so as to obtain a bit-rate which enables storing andrecording within the remaining storage capacity, and the resultantre-encoded broadcast program is stored and recorded. If the answer tothe determination as to whether the resolution conversion/frame ratedecimation is performed is YES, in step S8, the frame rate decimationand pixel reduction are performed, and the re-encoding processing isperformed upon the broadcast program to be stored in order to convertthe bit-rate thereof into a predetermined small bit-rate so that thebroadcast program can be stored within the remaining storage capacity ofthe storage medium. In this way, the required storage capacity of thestorage medium is compressed. Then, the re-encoded broadcast program isstored in the storage medium in the drive section 16. Examples of theresolution conversion processing may include a method for converting thenumber of horizontal pixels from 720 to 544 or 720 to 352 by using afilter and a method for converting the number of vertical pixels. In thelatter case, for example, a method for converting 480 vertical pixelsinto 240 pixels is known. The most simple method is to drop any field ofpixels formed of Top Bottom. A bit-rate conversion circuit 52 isconfigured by steps S4 to S8.

If it is determined in step S2 that the re-encoding processing is notperformed, in step S3, a broadcast program which has already been storedin the storage medium is deleted. Alternatively, the stored broadcastprogram is re-encoded to be compressed again in order to ensure a largerremaining storage capacity of the storage medium. A deletion circuit 54and a re-compression circuit 55 are configured by step S3.

In the deletion or re-compression processing in step S3, if two or morebroadcast programs have been already stored in the storage medium,priority is established in order to determine which of these broadcastprograms should be deleted or re-compressed.

In accordance with the method for establishing priority, a broadcastprogram which is temporarily stored in the storage medium and thencopied in other storage medium is set as the first priority fordeletion/re-compression processing. A broadcast program which is storedin the storage medium but is not played back at all even after apredetermined period of time freely set by a user is determined as thesecond priority. A broadcast program which is stored in the storagemedium and then played back by a user is set as the third priority. Abroadcast program which was stored in the storage medium at the earliestdate is set as the fourth priority. If a broadcast program satisfyingthese settings exists, the priority is established in an even furtherdetailed manner.

Although various methods for establishing the priority exist, an exampleof them will be illustrated in FIG. 3 in this embodiment. Referring toFIG. 3, a determination is made in step S20 as to whether or not two ormore stored broadcast programs are provided. If a plurality of storedbroadcast programs do not exist and only one broadcast program exists,in step S21, this broadcast program is deleted or re-compressed. Ifthere provided a plurality of broadcast programs, in step S22, whetheror not two or more broadcast programs that are copied in other storagemedium exist is determined. If only one broadcast program is copied inthe other storage medium, in step S23, the copied broadcast program isdeleted or re-compressed. If there provided a plurality of copiedbroadcast programs, a determination is made in step S24 as to whether ornot two or more broadcast programs recorded earlier than a date freelyset by a user exist. If there is provided only one broadcast programrecorded earlier than the date, in step S25, this broadcast program isdeleted or re-compressed. If there are provided two or more broadcastprograms recorded earlier than the set date, in step S26, adetermination is made as to whether or not there exist two or morebroadcast programs that have been once decoded and played back by a usersince their storage. If there is provided only one broadcast programthat has been decoded and played back since its storage, in step S27,this played back broadcast program is deleted or re-compressed. If thereexist two or more broadcast programs that have been played back once, instep S28, a broadcast program which was stored in the storage medium atthe earliest date is deleted or re-compressed.

Then, returning to step S1 in FIG. 2, if the answer to the determinationin step S1 is NO, i.e., if the broadcast program to be stored can besuccessfully stored in the storage medium and the remaining storagecapacity is larger than the required capacity of the storage medium andthus remains, in step S9, a determination is made as to whether or notthe broadcast program is stored and recorded with its original bit-rate,i.e., without converting the bit-rate of the broadcast program to bestored. If it is determined that the broadcast program is stored andrecorded with its original bit-rate, in step S10, this broadcast programis recorded. If it is determined that the broadcast program is notrecorded with its original bit-rate, in step S11, opposite to theprocessing in step S8 a frame interpolation and pixel interpolationprocessings are performed in order to convert the bit-rate into a largerbit-rate, so that high image quality is accomplished. Then, theresultant broadcast program is stored and recorded in the storagemedium. A second bit-rate changing circuit 53 is configured by step S11.This processing may be performed with the highest possibility when anMPEG4 image is recorded as an MPEG2 image. In accordance with the MPEG4image, unlike REC656 (30 Frame/sec, 720 horizontal pixels and 480vertical pixels) serving as a format obtained by digitizing a generalNTSC image, QCIF and CIF are provided as an image size and 15 Frame/secis provided as a frame rate. Such image size and frame rate areconverted by a filter into REC656 size, and then the MPEG4 image isre-encoded.

Returning to FIG. 1, the structure of the device for ensuring thestorage time for digital broadcast is continuously described. The MPEGdecoder 10 is a decoder for returning a compressed MPEG1, MPEG2 or MPEG4broadcast in a baseband. This decoder 10 may utilize, in addition toMPEG, JPEG, Motion JPEG or a compression system which is not defined byISO. The MPEG decoder 10 has the function of deleting padding bitsdescribed in step S5 in FIG. 2 as well as the function of decoding thebroadcast program stored in the storage medium in a baseband at the timeof playing back the same.

The image resolution/frame rate converter 9 illustrated in FIG. 1performs, as well as the reduction and interpolation processings forhorizontal resolution and the conversion of vertical resolution, theframe rate decimation and interpolation. An NTSC/PAL/SECAM encoder 8 isused to convert digital broadcast data into an analog signal in order tooutput to a television monitor, and converts, in order to correspond tobroadcasting systems for all over the world, an NTSC system used in theUnited States and Japan, a PAL system used in European countries andChina and an SECAM system used in the former USSR into the respectiveanalog systems. In the case of not standard broadcasting butbroadcasting with high resolution, an encoder corresponding to suchbroadcasting is required.

An NTSC/PAL/SECAM decoder 7 converts an analog signal from an analogtuner 15 into a digital signal, and corresponds to the NTSC system, thePAL system and the SECAM broadcasting. The analog broadcast is usuallydisturbed in a weak electric field, and a vertical or horizontal retraceline interval becomes long or short. This phenomenon may be occurred bya line input such as an input from VTR as well as an input from theanalog tuner 15. A TBC circuit 18 corrects this phenomenon. The TBCcircuit 18 usually accomplishes the correction by writing a oncedisturbed signal in a frame memory and reading out the signal. Dependingon the range of correction, the correction can be realized simply bywriting in a line memory. The output from the TBC circuit 18 iscompressed and encoded in the MPEG encoder 11. The compression system inthe MPEG encoder 11 may be, in addition to MPEG1, MPEG2 and MPEG4, JPEG,Motion JPEG and a compression system which is not defined by ISO. Thecompressed stream is written via the stream controller 6 into thestorage medium on the drive 16.

The stream controller 6, the image resolution/frame rate converter 9,the MPEG decoder 10, the MPEG encoder 11 and the microcomputer 14 areintegrated into one chip.

The MPEG encoder 11 and the MPEG decoder 10 are existing devicesrequired for recording/playing back an analog broadcast or an analogline input. In accordance with this embodiment, a record/play-backsystem for existing devices including such devices is utilized for thedecoding and encoding processings in steps S3, S7, S8 and S11illustrated in FIG. 2. Thus, in accordance with this embodiment,existing devices can be shared and thus the present invention can beprovided inexpensively.

1. A method for ensuring a storage time for digital broadcast at thetime of storing broadcast data transmitted in a state of being digitizedand compressed, comprising the steps of: monitoring a predeterminedrecording time for the broadcast and a bit-rate of the broadcast in thecompressed state and calculating a required capacity of a storagemedium; determining whether or not said calculated required capacity ofthe storage medium can be contained within an empty area or a freelylimited area of the storage medium; if said calculated required capacityof the storage medium cannot be contained as the result of saiddetermination, converting, in real time, said bit-rate of the broadcastinto a smaller one by decoding and then re-compressing the broadcastdata in the compressed state; and storing said converted broadcast insaid storage medium.
 2. The method for ensuring a storage time fordigital broadcast of claim 1 further comprising the steps of: if saidcalculated required capacity of the storage medium can be contained asthe result of said determination, determining whether or not said emptyarea or freely limited area of the storage medium remains; if said emptyarea or freely limited area remains, converting, in real time, saidbit-rate of the broadcast into a larger one; and storing said convertedbroadcast into said storage medium.
 3. The method for ensuring a storagetime for digital broadcast of claim 1 further comprising the step of: ifsaid calculated required capacity of the storage medium can be containedas the result of said determination, storing said broadcast in saidstorage medium without converting the bit-rate.
 4. The method forensuring a storage time for digital broadcast of claim 1, wherein saidstep of converting said bit-rate of the digitized broadcast into asmaller one is an operation of missing a part of said broadcast data. 5.The method for ensuring a storage time for digital broadcast of claim 1,wherein said step of converting said bit-rate of the digitized broadcastinto a smaller one is an operation of returning said broadcast in abaseband and performing a compression processing again for thebroadcast.
 6. The method for ensuring a storage time for digitalbroadcast of claim 5, wherein said operation of returning said broadcastin a baseband and performing a compression processing again for thebroadcast is performed by utilizing a system for compressing an analogbroadcast, decoding the compressed digital data and then playing backthe resultant data.
 7. The method for ensuring a storage time fordigital broadcast of claim 5, wherein said step of converting saidbit-rate of the digitized broadcast into a smaller one is an operationof returning said broadcast in a baseband, then performing imageresolution conversion or image frame rate conversion upon the broadcastand performing the compression processing again upon the resultantbroadcast.
 8. The method for ensuring a storage time for digitalbroadcast of claim 1, wherein said step of converting said bit-rate ofthe digitized broadcast into a smaller one is performed by selecting onefrom three kinds of operations, i.e., the operation of missing a part ofthe broadcast data, the operation of returning the broadcast in abaseband and performing a compression processing again upon thebroadcast and the operation of returning the broadcast in a baseband,then performing image resolution conversion or image frame rateconversion upon the broadcast and performing the compression processingagain upon the resultant broadcast.
 9. The method for ensuring a storagetime for digital broadcast of claim 1 further comprising the step of: ifsaid calculated required capacity of the storage medium cannot becontained, deleting a broadcast which has been already stored in saidstorage medium to ensure the required capacity of the storage medium.10. The method for ensuring a storage time for digital broadcast ofclaim 1 further comprising the step of: if said calculated requiredcapacity of the storage medium cannot be contained, re-compressing saidbroadcast which has been already stored in the storage medium to ensurethe required capacity of the storage medium.
 11. The method for ensuringa storage time for digital broadcast of claim 10, wherein said step ofre-compressing said broadcast which has been already stored in thestorage medium is performed by the operation of missing a part of thestored broadcast data, the operation of returning the stored broadcastin a baseband and performing a compression processing again upon thebroadcast or the operation of returning said stored broadcast in abaseband, then performing image resolution conversion or image framerate conversion and performing a compression processing again upon thebroadcast.
 12. The method for ensuring a storage time for digitalbroadcast of claim 9, wherein when a plurality of already storedbroadcasts exist in the storage medium, a broadcast that a set period oftime or longer has passed since its storage will be deleted.
 13. Themethod for ensuring a storage time for digital broadcast of claim 9,wherein when a plurality of already stored broadcasts exist in thestorage medium, a broadcast which has been decoded at least once sinceits storage will be deleted.
 14. The method for ensuring a storage timefor digital broadcast of claim 9, wherein when a plurality of alreadystored broadcasts exist in the storage medium, a broadcast which isstored in the storage medium and then copied in other storage mediumwill be deleted.
 15. The method for ensuring a storage time for digitalbroadcast of claim 9, wherein when a plurality of already storedbroadcasts exist in the storage medium, a broadcast which was stored atthe earliest date will be deleted.
 16. The method for ensuring a storagetime for digital broadcast of claim 9, wherein when a plurality ofalready stored broadcasts exist in the storage medium, a broadcast thata set period of time or longer has passed since its storage, a broadcastwhich has been decoded at least once since its storage, a broadcastwhich is stored in the storage medium and then copied in other storagemedium or a broadcast which was stored at the earliest date will bedeleted, and these broadcasts are selected in accordance with apredetermined priority.
 17. The method for ensuring a storage time fordigital broadcast of claim 10, wherein when a plurality of alreadystored broadcasts exist in the storage medium, a broadcast that a setperiod of time or longer has passed since its storage will bere-compressed.
 18. The method for ensuring a storage time for digitalbroadcast of claim 10, wherein when a plurality of already storedbroadcasts exist in the storage medium, a broadcast which has beendecoded at least once since its storage will be re-compressed.
 19. Themethod for ensuring a storage time for digital broadcast of claim 10,wherein when a plurality of already stored broadcasts exist in thestorage medium, a broadcast which is stored in the storage medium andthen copied in other storage medium will be re-compressed.
 20. Themethod for ensuring a storage time for digital broadcast of claim 10,wherein when a plurality of already stored broadcasts exist in thestorage medium, a broadcast which was stored at the earliest date willbe re-compressed.
 21. The method for ensuring a storage time for digitalbroadcast of claim 10, wherein when a plurality of already storedbroadcasts exist in the storage medium, a broadcast that a set period oftime or longer has passed since its storage, a broadcast which has beendecoded at least once since its storage, a broadcast which is stored inthe storage medium and then copied in other storage medium or abroadcast which was stored at the earliest date will be re-compressed,and these broadcasts are selected in accordance with a predeterminedpriority.
 22. A device for ensuring a storage time for digital broadcastat the time of storing a digitized and then transmitted broadcastcomprising: a required capacity calculation circuit for monitoring apredetermined recording time of the broadcast and a bit-rate of thebroadcast and calculating, in real time, a required capacity of thestorage medium; a determination circuit for determining whether or notthe required capacity of the storage medium calculated in said requiredcapacity calculation circuit can be contained within an empty or freelylimited area in the storage medium; and a bit-rate conversion circuitfor, if said calculated required capacity of the storage medium cannotbe contained as the result of said determination by said determinationcircuit, converting, in real time, said bit-rate of the broadcast into asmaller one and storing the converted broadcast in said storage medium.23. The device for ensuring a storage time for digital broadcast ofclaim 22, wherein said determination circuit further determines, if saidcalculated required capacity of the storage medium can be contained,whether or not said empty or freely limited area of the storage mediumremains, and said device further comprising: a second bit-rate changingcircuit for, if the empty or freely limited area remains as the resultof said determination by said determination circuit, converting, in realtime, said bit-rate of the broadcast into a larger one and storing theconverted broadcast in said storage medium.
 24. The device for ensuringa storage time for digital broadcast of claim 22, wherein said requiredcapacity calculation circuit comprising: a determination circuit forwriting an inputted broadcast stream in a storage memory and determiningsaid bit-rate of the broadcast on the basis of its storage time andstorage amount.
 25. The device for ensuring a storage time for digitalbroadcast of claim 22 further comprising: a deletion circuit for, ifsaid calculated required capacity of the storage medium cannot, becontained as the result of determination by said determination circuit,deleting a broadcast which has been already stored in the storagemedium.
 26. The device for ensuring a storage time for digital broadcastof claim 22 further comprising: a re-compression circuit for, if saidcalculated required capacity of the storage medium cannot be containedas the result of determination by said determination circuit,re-compressing a broadcast which has been already stored in the storagemedium.
 27. The device for ensuring a storage time for digital broadcastof claim 22, wherein said required capacity calculation circuit, saiddetermination circuit and said bit-rate conversion circuit areintegrated onto a chip.